In the etching of material on a substrate, particularly in plasma etching, it is necessary to etch openings through certain material in such a way as to accurately control the opening size and also accurately control the edge profiles and undercutting of the etched openings. One example of this is etching polysilicon on an SiO.sub.2 base in the manufacture of semiconductor chips. The openings are etched through a mask and the edge contour should terminate at the substrate in vertical alignment with the edge of the mask.
In conventional isotropic etching, such as by plasma etching, the etch takes place laterally as well as vertically leaving an opening which has curved side walls. It is desirable that the etch terminate as soon as the material has been etched through to the substrate so that the edge of the bottom of the wall is exactly in vertical alignment with the opening in the mask. This is no particular problem if the material is of uniform thickness such that the etching at all locations is completed at the same time. However, there is normally significant variations in the thickness of the material being etched. Therefore, if the etch is continued to completion at the thickest part, there will be significant detrimental undercutting or etch bias of the thinner parts where the vertical etching terminates sooner, but the horizontal etching continues. Using anisotropic etching to eliminate any undercutting will not solve the problem since the walls will turn out vertical rather than with the desired curvature.